The aim of this research is to elucidate the control of the differentiation of a multipotent stem cell, the interstitial cell (i-cell) of hydra. The approach is to alter the cell composition of intact animals and study the i-cell division and differentiation behavior. The cell composition can be altered by the selective deletion of cell types in intact animals and with a aggregation technique. This involves dissociating hydra into single cells and forming cell aggretates which develop into normal animals. Separation of cell types will permit construction of aggregates (hence animals) of selected cell composition. The pattern of i-cell behavior will be analyzed quantitatively as methods have been developed for quantitatively measuring all cell populations in hydra, as well as examining quantitatively the behavior of a cohort of isotopically-labeled cells with autoradiography. The objects of the proposal are to: (1) study the effects of differentiation product cell population and the i-cell population size on i-cell behavior; (s) study the basis of position-dependent differentiation of i-cells; (3) study the reversibility of commitment and the phase of the cell cycle during which commitment takes place; (4) continue to develop cell separation procedures.